Telit Wireless Solutions GM862-GPS - Manual (Page 37)

GM862-GPS Hardware User Guide
1vv0300728 Rev. 0 - 27/04/06
Reproduction forbidden without Telit Communications S.p.A. written authorization - All Right reserved page 37 of 55
The buffer bandwidth at -3dB shall be 4KHz.
Note that the biasing of the operational amplifier is given for the inverting amplifier by the series divider
R714-R715. The 100nF capacitor C719 is needed to filter the noise that could be coupled to that
divider. For the not inverting operational amplifier the biasing is given by a different divider R715-R717
with the capacitor C720 and through a series resistor R718 of 470KΩ.
TIP: example of calculation.
Llet's assume you have a microphone with a sensitivity of -45dBV
rms
/Pa and you want to use it in 2nd
differential microphone path (“Mic_HF” inputs) in "normal spoken" conditions at acoustic pressure
of -4.7dBPa.
As reported at page XX , the electrical level output from the microphone will be :
but we have to consider 20dB loss due to the higher distance from the mouth of the talker ( 50cm ) .
corresponding to
In order to have a signal of 1 mV
rms
at the “Mic_HF” inputs , as suggested at TIP “environment
consideration “,
the buffer must have a gain
or +10 dB
Keeping in mind that “ balancing the line will double the signal”, to calculate the resistor values assign
half of required gain G
A
to each amplifier section . And therefore G
S
=1,5times (or +3,52dB) .
Choosing as 10kΩ as the input resistance , the corresponding values for the resistors on the buffer will
be :
R711 = G
S
* R708= 1.5*10 =15 kΩ
R719 = (G
S
-1) * R720 = (1.5 -1)*10 =5 kΩ
The commercial values of 15kΩ and 5.6kΩ be accepted .
As a consequence of the assigned values of the resistors, the nominal values of C726 and C727 are
:
MicLevel = ( -45) + (-4.7) = -49.7 dBV
rms
MicVoltage = 10 ( -69.7 / 20 ) = 0,33* 10 -3
MicLevel = ( -49.7) + (-20) = -69.7 dBV
rms
G
A
= “Mic_HF /MicVoltage = (1*10 -3 )/(0,33*10
3